Apparatus for stabilizing component elution times in gas chromatographs

ABSTRACT

A peak stabilizer in accordance with this invention controls the peak retention time during an analysis, by gas chromatograph, for example, so that this time will be the same for all determinations. This allows a peak to be gated or picked for process control or read-out without the undesirable possibility that it would drift beyond the time limits of the gate. If a peak in the chromatogram, for example, is sampled on the leading and trailing slopes at predetermined times, a comparison of these voltages will indicate if the peak is early, late or at its right time. The carrier gas flow or other rate control parameter may be automatically changed to bring the peak back to the proper retention time.

United States Patent [191 Martin APPARATUS FOR STABILIZING COMPONENT ELUTION TIMES 1N GAS CHROMATOGRAPHS [75] Inventor: Fred D. Martin, Angleton, Tex.

[73] Assignee: The Dow Chemical Company,

Midland, Mich.

[22] Filed: Jan. 12, 1971 [21] Appl. No.: 105,893

[4 1 Feb. 6, 1973 Primary Examiner.lames J. Gill Assistant Examiner-C. E. Snee, lll Attorney-Griswold & Burdick and Earl D. Ayers [57] ABSTRACT A peak stabilizer in accordance with this invention controls the peak retention time during an analysis, by gas chromatograph, for example, so that this time will be the same for all determinations. This allows a peak to be gated or picked for process control or read-out without the undesirable possibility that it would drift beyond the time limits of the gate. If a peak in the chromatogram, for example, is sampled on the leading and trailing slopes at predetermined times, a comparison of these voltages will indicate if the peak is early, late or at its right time. The carrier gas flow or other rate control parameter may be automatically changed to bring the peak back to the proper retention time.

3 Claims, 2 Drawing Figures 6 as chroma 1 09/0 06 T/rner Ca 17' fer 9675 APPARATUS FOR STABILIZING COMPONENT ELUTION TIMES IN GAS CHROMATOGRAPHS BACKGROUND OF THE INVENTION This invention relates to peak stabilizer apparatus, and particularly to apparatus for controlling the retention of a peak during an analysis, as by gas chromatography, for example.

Small changes in carrier gas flow or column temperature in a gas chromatograph analyzer will cause the time the resolved sample component peaks emerge from the separating column to vary. Normal temperature control and gas flow control provide acceptable retention time reproducibility for most applications, however more elaborate controls may not be adequate where peak information is automatically selected for special readout or process control. In these cases, the peak time gate must be set to identify the peak or peaks of interest and it is apparent that if the peak retention time varies, it may drift beyond the time limits of the gate.

Another problem exists if the peak retention time changes between analyses. It becomes necessary to measure peak area rather than peak height to obtain accurate quantitative determinations. When a given amount of sample is retained in the separating column a longer period of time, the emerging peak will be broader and shorter.

Accordingly, a principal object of this invention is to provide improved apparatus for stabilizing peak retention times in the output signal of an analytical instrument.

Another object of this invention is to provide improved apparatus for use with a gas chromatograph to stabilize peak retention time.

In accordance with this invention a peak in the chromatogram is sampled on the leading and trailing slopes at predetermined times, these voltages, when compared, will provide an output which is neutral, negative or positive and thus indicate if the peak is at its right time, is early or is late. The carrier gas flow may be automatically changed to bring the peak back to the proper retention time in accordance with the polarity and magnitude of the output of the apparatus.

The invention, as well as other objects and advantages thereof, will best be understood when the following detailed description is read in connection with the accompanying drawing, in which:

FIG. 1 is a diagrammatic view of apparatus in accordance with this invention, and

FIG. 2 is a representation of output peaks of an analytical instrument which is used to illustrate the operation of the apparatus of FIG. 1.

Referring to FIG. 2, there is shown a typical chromatogram composed of peaks A, B, C and D. A major peak that is always present and emerges late is selected as the control peak for correcting the flow rate through the chromatography device for the next analysis. Peak D meets these requirements.

Operation may be understood by following the circuit in FIG. 1. The sequence is controlled by a timer 110 that operates switches 12, 14,98, and 100 and is initiated by a master timer (not shown). The output voltage E of gas chromatograph is sampled at time X (FIG. 2) by closing contact F on switch 12 for a short period of time. This value is stored by the hold amplifier 28 which has inverted polarity diodes 22, 24 coupled across its input positive and negative inputs. The input value from contact F is applied to the positive input of amplifier 28. Capacitor l8 and resistor 26 are coupled between ground and the positive and negative inputs, respectively, of amplifier 28. Then the output voltage E is sampled at time Y (see FIG. 2) by closing contact G of switch 14. The value is stored by amplifier 42 which is similar to amplifier 28. The two values of E at times X and Y are compared by amplifier 54. Amplifiers 42 and 54 each have inverted diodes 38,40 and 50,52 respectively, connected in parallel across their positive and negative inputs.

Amplifier 42 has a capacitor 20 and resistor 66 connected across its inputs and to ground 16 the same as capacitor 18 and resistor 26 are connected to amplifier 28. Amplifiers 28 and 42 each have a resistor 30,44 respectively coupled between its output and ground and a feed back resistor 36,46 respectively coupled between its output and'negative input.

The output of amplifier 28 is coupled through resistor 32 and contact 34 of switch S to the negative input of amplifier 54, while the output of amplifier 42 is coupled through resistor 48 to the positive input of amplifier 54. Feed back from the output of amplifier 54 to its negative input is accomplished through resistor 36.

An internal trim adjustment composed of resistor 64 and variable resistor having a movable contact 62 is shown coupled to the amplifier 54. Similar trim adjustments, not shown, may be provided for amplifiers 28 and 42.

The output of amplifier 54 is coupled to the base of transistors 102, 104 whose emitters are coupled through resistors 106, 107, respectively, to ground.

The collector of transistor 102 is coupled to a negative voltage through relay coil 72 which controls the opening and closing of switch 74.

The collector of transistor 104 is coupled to a positive voltage source through relay coil 109 which controls the opening and closing of switch 84.

A part of the output of amplifier 54 is coupled from the movable contact 58 of variable resistor 56 to the bases of transistors 108, 86 whose emitters are coupled to ground through resistors 75,90, respectively.

The collectors of transistors 108,86 are coupled respectively to negative and positive voltage sources through relay coils 68,78 which control the opening and closing of switches 70,80 respectively. Switches 70,80 are connected in parallel with switches 74,84, respectively.

A reversible electric motor 96 has the junction of its two windings coupled to one side of a suitable AC. power source. The other side of the AC. power source is coupled, respectively, through switches 98, 100, 80, 84 or 98, 100, 70, 74 to an end of one of the windings of the motor 96 which is mechanically coupled to the valve 94 in the carrier fluid line feeding the gas chromatograph 10.

For the sake of simplicity, a timer 110 is shown which controls switches 12,14. 34, 98, 100 in suitable operating sequence.

In operation, the timer 1 l0 first closes switch 12 at F to couple the output of the chromatograph 10 to the amplifier to store the value of point X (FIG. 2) on peak D. The timer then couples the output of the chromatograph across switch 14 to amplifier 42 to store the value of point Y of peak D.

Then, with switch S (34), and switches 98 and 100 usually closed, the values of X and Y are compared in amplifier 54 and the resultant output of that amplifier is coupled to the transistors 102, 104, 108, 86 controlling switches 74, 84, 70 and 80, respectively.

The switch 100 is closed for a shorter time than is switch 98, used for making so-called large timing corrections in carrier fluid flow through the chromatograph 10.

The output of amplifier 54 is either positive, negative or zero.

If the output is at or near zero, none of the switches 74, 84, 70 or 80 will be operated.

If the output of the amplifier S4 is positive or negative, the appropriate relay switch 74 or.84 will be closed and, during the time the switch 100 is energized, the motor 96 will be energized to correct the flow of carrier fluid to the chromatograph 10 by a specific amount.

If the amplitude of the output signal from the amplifier S4 exceeds a predetermined amount (as selected from the movable contact 58), the switch 70 or 80 closes, allowing a larger correction to be made, since the switch 98 is closed for a longer time than is the switch 100.

The switch 34, when open, leaves one input to the amplifier 54 floating, effectively preventing any output from the amplifier 54 and possible energization of relays until the actual comparison of X and Y signals are to be made. The switch S (34) could be eliminated if power economy were not desired.

in view of the time delay between successive cycles of operation, values of previous X and Y signals dissipate so that the new signals are the result only of the new" X and Y values.

What is claimed is:

l. A system for stabilizing the peak retention time in fluid chromatographs, comprising means including a capacitor and an amplifier for sequentially measuring and storing amplitude values at points along opposite slopes of a selected peak signal, signal comparison amplifier means for providing an output signal which is a function in amplitude and polarity of the difference between said stored values, means for applying said output signal to polarity and amplitude sensitive switching means, said switching means coupling a power source to reversible motor drive means, said drive means being coupled to a valve for controlling the flow of carrier fluid to said chromatograph as a function of the polarity and amplitude of said signal.

2. A system in accordance with claim 1, wherein timer means controls the time interval during which said drive means may be actuated.

3. A system in accordance with claim 1, wherein said output signal is split and is coupled to separate polarity sensitive switching means, each of said switching means being coupled to a separate electrical winding of said reversible motor drive means, said power source being coupled to said switching means at predetermined times. 

1. A system for stabilizing the peak retention time in fluid chromatographs, comprising means including a capacitor and an amplifier for sequentially measuring and storing amplitude values at points along opposite slopes of a selected peak signal, signal comparison amplifier means for providing an output signal which is a function in amplitude and polarity of the difference between said stored values, means for applying said output signal to polarity and amplitude sensitive switching means, said switching means coupling a power source to reversible motor drive means, said drive means being coupled to a valve for controlling the flow of carrier fluid to said chromatograph as a function of the polarity and amplitude of said signal.
 1. A system for stabilizing the peak retention time in fluid chromatographs, comprising means including a capacitor and an amplifier for sequentially measuring and storing amplitude values at points along opposite slopes of a selected peak signal, signal comparison amplifier means for providing an output signal which is a function in amplitude and polarity of the difference between said stored values, means for applying said output signal to polarity and amplitude sensitive switching means, said switching means coupling a power source to reversible motor drive means, said drive means being coupled to a valve for controlling the flow of carrier fluid to said chromatograph as a function of the polarity and amplitude of said signal.
 2. A system in accordance with claim 1, wherein timer means controls the time interval during which said drive means may be actuated. 